Cross-coupling of alkynes

Zhao and Larock have described the synthesis of carbazoles, indoles, and dibenzofurans 118 via a Ic type cyclization that follows a sequence of Pd-catalyzed cross-coupling of alkynes and aryl iodides 116, then nitrogen-directed palladium migration to an arylpalladium intermediate 117 that undergoes an intramolecular Mizoroki-Heck ring closure <06JOC5340>. 

A new type of triaryl phosphine-functionalized imidazolium salt containing cations such as (6) has been prepared. Palladium complexes of (6) generated in situ have been used successfully in Heck-type reactions of aryl halides with acrylates and of 4-bromotoluene with styrene derivatives.34 The first Heck-type reaction of aryl halides with allenes has been reported. 1,3-Double arylations were observed with 3-substituted-l,2-allenyl sulfones, while 1-monoarylation was favoured with 3,3-disubstituted-l,2-allenyl sulfones.35 It has been shown that the a-arylation of methane-sulfonamides (7) may be achieved using palladium catalysis reaction proceeds through the sulfonamide enolates.36 It is also reported that palladium cross-coupling of alkynes with /V - (3 - i odophe n y I an i I i ncs) may lead to the formation of substituted carbazoles.37... 

Homo- and cross-couplings of alkyne promoted by metallic catalysts are versatile reactions for carbon-carbon bond formation and have been utilized to synthesize functional hyperbranched polymers. 

An effective cross-coupling of alkynes using silicon-tethered Fe(CO)s-promoted cyclocarbonylation was shown to provide a seven-membered ring dialkoxysilane 302, which subsequently upon treatment with Me3NO in acetone at 0°C was converted to cyclopentadienones 303 with variable substituents (Scheme 54) <2002OL2837>. 

Zhao and Larock have introduced a convenient method for the preparation of substituted dibenzofurans as well as carbazoles and indoles by palladium-catalyzed cross-coupling of alkynes and appropriately substituted aryl iodides. These reactions proceed by carbopalladation of the alkyne, heteroatom-directed migration of palladium from a vinyl to the adjacent aryl position, and ring closure via intramolecular arylation (Scheme 87) <2006JOC5340>. 

Although copper-catalyzed homocoupling of alkynes and Ni(0)-catalyzed homocoupling of aryl halides were well known, Cadiot and Chodkiewicz gave first-time synthetically more useful copper-catalyzed cross-coupling of alkynes. 

Cationic palladium-catalyzed addition of arylboronic acids to nitriles for the formation of benzo[h]furans was reported <06OL5987>, an example of which is illustrated in the following scheme. The palladium-catalyzed cross coupling of alkynes with appropriately substituted aryl iodides for the synthesis of substituted dibenzofurans in moderate to excellent yields was also achieved <06JOC5341>. The benzo[fc]furan core of heliannuls G and H were constructed by a palladium-catalyzed Ji-allyl cyclization reaction <06TL7353>. The palladium-catalyzed oxidative activation of arylcyclopropanes was applied to the synthesis of 2-substituted benzo[Z>]furans <06OL5829>. 

In another copper-catalyzed reaction, cross-coupling of alkynes with phosphi-ne-boranes was followed by surprising oxidation to yield ketones (Scheme 69) [122]. The active species was proposed to be a copper phosphido-borane complex, formed by proton transfer to a Cu-OH group. Formation of a Cu-acetylide followed by P-C reductive elimination would then yield a phosphino-alkyne, whose subsequent Cu-mediated air oxidation yields the ketone. 

The coupling products are obtained by one-pot hydrosilylation/cross-coupling of alkynes. Pt-catalyzed hydrosilylation of alkyne with tetramethyldisiloxane (52) generates the alkenylsilane 53, which is coupled with 4-iodoanisole (2) using ligandless Pd catalyst and TBAF at room temperature to afford 46 in 84 % yield [103]. 

Scheme 4.47 One-pot hydrozirconation/transmetallation/Pd-catalyzed cross-coupling of alkynes [61 cj.

Ynamides can be easily synthesized by metal-catalyzed amidative cross-coupling of alkynes (Scheme 4.50). Other alternatives employ lithiated amides with al-kynyliodonium salts [196], or metal-catalyzed coupling of amides with alkynyl bromides [196], potassium alkynyltrifluoroborates [197], 1,1-dihalo-1-alkenes [198], or propylic acids [199]. 

Scheme 4.50 Copper-catalyzed amidative cross-coupling of alkynes...

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